SHORT COMMUNICATION
SHORT COMMUNICATION
Plant Signaling & Behavior 8:2, e23123; February 2013; © 2013 Landes Bioscience
Circadian control of jasmonates and salicylates
The clock role in plant defense
Danielle Goodspeed, E. Wassim Chehab, Michael F. Covington† and Janet Braam*
Biochemistry and Cell Biology; Rice University; Houston, TX USA
†
Current affiliation: Department of Plant Biology; College of Biological Sciences; University of California; Davis, CA USA
Keywords: Arabidopsis thaliana, jasmonic acid, salicylic acid, Trichoplusia ni, circadian rhythm, plant-insect interaction, herbivory,
biotic stress, plant resistance
Plants have evolved robust mechanisms to perceive and respond to diverse environmental stimuli. The plant
phytohormones jasmonates and salicylates play key roles in activating biotic stress response pathways. Recent findings
demonstrate that basal levels of both jasmonates and salicylates in Arabidopsis are under the control of the circadian clock
and that clock-controlled jasmonate accumulation may underlie clock- and jasmonate-dependent enhanced resistance
of Arabidopsis to Trichoplusia ni (cabbage looper), a generalist herbivore. Here we summarize these findings and provide
further evidence that a functional plant circadian clock is required for optimal herbivore defense in Arabidopsis. When
given a choice to feed on wild-type plants or arrhythmic transgenics, T. ni prefer plants lacking robust circadian rhythms.
Altogether these data provide strong evidence for circadian clock enabling anticipation of herbivore attack and thus
contributing to overall plant fitness.
Nearly all organisms have evolved an internal clock, which
influences diverse behaviors that exhibit 24-h, or circadian,
rhythms.1,2 The circadian clock is thought to be advantageous
to diverse organisms because it provides the ability to anticipate
the predictable daily and seasonal changes that occur in the environment as a consequence of the rotation of the earth. A few
examples of clock-dependent anticipatory daily behaviors are well
known. Some plant leaves turn to expose surfaces toward the east
during the night to anticipate the rising sun.3 The effect of the
clock on internal physiological processes may be often underappreciated because experiments must be designed in ways to
reveal the impact of the dimension of time. Given the widespread
effect of the clock on plant gene expression—with approximately
one-third of Arabidopsis transcripts showing circadian regulation
of accumulation,4 it is predicted that the clock has broad impact
on plant physiology.
Recent demonstration that over 40% of genes known to be
upregulated in expression by wounding also show circadian regulation of transcript accumulation in unwounded plants5 led to
the question of the physiological relevance of these massive daily
fluctuations in gene expression. In a recent study,6 we sought
to test the hypothesis that the circadian clock enables plants to
anticipate daily fluctuations in potential herbivore behavior by
mounting defenses prior to likely attack.
Remarkably, Arabidopsis plants, entrained in light-dark cycles
and then placed in constant conditions, to remove potential
behaviors influenced by changes in light and reveal only those
behaviors controlled by the clock, accumulated two major
defense hormones, jasmonates and salicylates, with distinct circadian patterns.6 In unwounded plants, jasmonate levels peaked
in the middle of subjective day and salicylate levels peaked in
the middle of subjective night. Thus, in unwounded, uninfected
plants, hormone levels fluctuated with daily rhythmicity, perhaps
in anticipation of attack.
Indeed, the circadian clock function is critical for Arabidopsis
resistance to herbivory. The generalist herbivore, Trichoplusia ni
larvae (commonly known as cabbage loopers) demonstrated a
circadian feeding behavior, with peak feeding occurring during
the middle to end of subjective day.6 When Arabidopsis plants
and the cabbage loopers were entrained with in-phase light-dark
cycles, the plants demonstrated substantial resistance. Cabbage
looper performance was enhanced and Arabidopsis resistance was
reduced if the plants and insects were entrained with opposite
light-dark cycles. As a consequence of these opposite entrainment conditions, when the plants were experiencing subjective
night (and therefore not accumulating jasmonates), the insects
were experiencing subjective day and therefore were highly
active in feeding. These experiments demonstrated that proper
*Correspondence to: Janet Braam; Email: [email protected].
Submitted: 12/03/12; Accepted: 12/04/12
http://dx.doi.org/10.4161/psb.23123
Citation: Goodspeed D, Chehab E, Covington M, Braam J. Circadian control of jasmonates and salicylates: The clock role in plant defense. Plant Signal Behav
2013; 8:e23123; PMID: 23299428; http://dx.doi.org10.4161/psb.23123.
www.landesbioscience.com
Plant Signaling & Behavior
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Figure 1. In a choice experiment, T. ni prefer to feed on arrhythmic
transgenics over wild-type plants. Plants were entrained for 3 weeks
and loopers were entrained for 3 d in 12-h light/dark cycles at 22C then
both plants and insects were moved to constant light conditions. After
24 h of constant light, plants and loopers were co-incubated for 72 h.
Loopers were evenly distributed between the two rows of plants. (A)
Photograph showing the alternative arrangement of 16-plant plots of
arrhythmic CCA1-OX plants and wild type after feeding by T. ni cabbage
loopers. (B and C) Close up images from (A) showing representative
plants and damage by T. ni feeding. (D) Relative percent area lost from
plant tissue illustrated in (A), determined using ImageJ software (National Institutes of Health). Means ± SE; n = 3; * p < 0.05; unpaired t-test.
Experiment was repeated 3 times with similar results.
entrainment with the environment, in this case entrainment with
cabbage loopers, enhances plant herbivory resistance.
We also demonstrated that both the circadian clock function and the jasmonate pathway are required for the enhanced
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resistance conferred by in-phase entrainment of the plants.6
Plants that are arrhythmic due to clock dysfunction and mutants
that cannot accumulate active jasmonates or lack the jasmonate
co-receptor failed to demonstrate the in-phase enhancement of
herbivore resistance. Furthermore, the data presented indicate
that the clock-controlled increase in jasmonate accumulation preceded that of the clock-controlled increase in herbivore feeding
behavior.6 The finding that plant phytohormone accumulation
begins prior to the relevant insect behavior is consistent with the
plant circadian clock enabling anticipation of predictable biotic
stress.
To further test the importance of circadian clock function in
herbivore resistance, we conducted herbivore choice experiments,
in which cabbage loopers were given the choice to feed upon either
clock-defective CCA1-OX transgenic plants7 or wild type (Col-0).
Two identical trays of six sets of plants (16 plants per plot) were
arranged such that the two genotypes were interspersed, with
wild type at top left, top right, and bottom center, and CCA1-OX
at top center, bottom left, and bottom right (Fig. 1A). Both plant
genotypes and the cabbage loopers were entrained in coincident
12-h light-dark cycles before co-incubation of insects with plants
at constant light. One tray of plants was used as a control in that
the plants were not exposed to the cabbage loopers. With the
second tray of plants, 15 T. ni cabbage loopers were evenly distributed at the center interface between the top and bottom rows
of plants and allowed to feed for 72 h. We compared the amount
of tissue remaining among plants exposed to the cabbage loopers
to that of plants not exposed to the insect herbivore to calculate
percent tissue lost. Figure 1 reveals that a significantly greater
percentage of tissue is lost from the CCA1-OX transgenic plants
than wild type due to herbivory. Greater tissue damage among
the CCA1-OX plants than the wild type is apparent in photographs of the plants (Fig. 1B and C). Quantitation of tissue loss
verifies that the herbivore damage was significantly greater on the
arrhythmic plants (CCA1-OX ) than on the wild type controls
(Fig. 1D). Therefore, when cabbage loopers are given a choice
between wild-type plants and plants with dysfunctional circadian clocks, the arrhythmic plants are preferred over the wildtype control. Thus, a functional Arabidopsis circadian clock is
clearly advantageous for plant cabbage looper resistance.
In conclusion, the plant circadian clock can significantly
enhance and is required for robust herbivore resistance. The demonstration that two major plant defense hormones, jasmonates
and salicylates, fluctuate with daily rhythms and finding that the
essential role the jasmonate pathway in clock-dependent looper
resistance in Arabidopsis are strong evidence that plants use the
circadian clock to anticipate attack and prepare in advance to
best fight off attackers.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Acknowledgments
This material is based upon work supported by the National
Science Foundation under Grant No. MCB 0817976 Grant to
J.B.
Plant Signaling & Behavior
Volume 8 Issue 2
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